X-ray systems are commonly employed for such applications as the inspection of containers by forming a quantitative image of the intensity of the x-rays transmitted through the containers. To form the image it is necessary that the x-ray beam be characterized by photons of sufficient energy and a flux of sufficient intensity (photons per unit area per unit time) that the beam may penetrate the most heavily loaded portions of the container undergoing inspection, and deliver sufficient intensity to the detectors of the transmitted beam that adequate signal-to-noise may be achieved during the scan time so that images of adequate quality may be obtained. For a given energy distribution of x-rays, the flux is tantamount to the x-ray power in the beam.
Safety of personnel requires that ambient levels of x-ray radiation fall below prescribed limits. Under the inspection scenario described above, a heavily loaded container gives rise to the lowest levels of ambient radiation since the contents of the container strongly absorb the x-ray beam, and minimal x-ray energy is scattered into the ambient environment of the x-ray system. However, if the container is only lightly loaded, ambient radiation may increase substantially.
If both the characteristic energy of the x-ray beam and its power are maintained constant, then the system must be shielded to account for the empty-container "worst case" level of ambient radiation, substantially adding to the requisite weight and cost of the system. In some cases, the requisite weight of the shielding drives the energy and power of the x-ray generator and hence the performance of the system. This may be the case, for example, for hand-held portable x-ray systems.
Current cargo systems operate at a maximum energy and at power levels complying with U.S. Government radiation requirements of a so-called "Cabinet System." That classification allows mobile inspection systems, for example, to be used with minimal restrictions in congested areas, a feature so highly desirable as to be a requirement that the Government imposes on the mobile x-ray systems it purchases. In order to use higher peak energies than are commonly employed, the x-ray intensity may modulated so that the highest intensity consistent with a Cabinet System is always maintained. In that way, a heavily loaded truck may be inspected with a beam intensity that is orders of magnitude greater than that used (or necessary) when the inspected truck is lightly loaded.
The control of x-ray beam intensity is well known in the art, and is used, for example, for reducing the dynamic range of intensity incident on radiation detectors in the face of intervening material, and for reducing, or evening out, the heat load on the anode of the x-ray source.